HUMAN BIOLOGY

16 Chapter 2

atom Smallest unit having
the properties of a given
element.

element Pure substance
that cannot be broken down
to another substance by
ordinary chemical or physi-
cal techniques.

atoms and elements

n Pure substances called elements are the basic raw material

of living things.

n Each element consists of one type of atom.

n The parts of atoms determine how the molecules

of life are put together.

n Link to Life’s organization 1.3

elements are pure substances

Like all else on Earth, your body consists of chemicals.

Some of them are solids, others liquid, still others gases.

Each of these chemicals consists of

one or more elements. An element

is a pure substance: that is, it can-

not be broken down to another

substance by ordinary physical

or chemical techniques. There are

more than ninety natural elements

on Earth, and scientists have created

many other artificial ones.

Overall, organisms consist mostly of four elements: oxy-

gen, carbon, hydrogen, and nitrogen. The human body also

contains some calcium, phosphorus, potassium, sulfur,

sodium, and chlorine, plus many different trace elements

(Figure 2.1). A trace element is one that makes up less than

0.01 percent of body weight. Trace elements are extremely

important, however. For example, without the trace ele-

ment iron, your red blood cells can’t carry oxygen. The

body’s chemical makeup is finely tuned. Many trace ele-

ments found in our tissues—such as arsenic, selenium, and

fluorine—are toxic in amounts larger than normal.

Atoms of the same or different elements can combine

into molecules—the first step in biological organization.

Molecules in turn can combine to form larger structures,

as described shortly.

atoms are composed of smaller particles

An atom is the smallest unit that has the properties of

a given element. A million could fit on the period at the

end of this sentence. In spite of their tiny size, however,

all atoms are composed of more than one hundred kinds

of subatomic particles. The ones we are concerned with in

this book are protons, electrons, and neutrons, illustrated

in Figure 2.2.

All atoms have one or more protons. These particles

carry a positive charge, marked by a plus sign (p^1 ). Atoms

also have one or more neutrons, which have no charge.

Neutrons and protons make up the atom’s core, the atomic

nucleus. Electrons move around the nucleus, in the space

that occupies 99.99 percent of the atom’s volume. Electrons

have a negative charge, which we write as e^2. An atom usu-

ally has equal numbers of electrons and protons.

Each element is assigned an “atomic number,” which

is the number of protons in its atoms. Elements also have

a “mass number”—the sum of the protons and neutrons

in the nucleus of their atoms. Appendix II of this textbook

has charts of the elements and of the atomic numbers of the

common elements in living things.

Figure 2.1 Everything in the biosphere, from humans

to the Earth’s crust, is made of elements.

Cape Verde National Institute of Meteorology and Geophysics and the U.S. Geological Survey

Oxygen 46.6

Silicon 27.7

Aluminum 8.1

Iron 5.0

Calcium 3.6

Sodium 2.8

Potassium 2.6

Magnesium 2.1

Other elements 1.5

Earth’s crust

Oxygen 65

Carbon 18

Hydrogen 10

Nitrogen 3

Calcium 2

Phosphorus 1.1

Potassium 0.35

Sulfur 0.25

Sodium 0.15

Chlorine 0.15

Magnesium 0.05

Iron 0.004

Iodine 0.0004

Human

2.1

Figure 2.2 Animated! Atoms consist of subatomic particles.

This model does not show what an atom really looks like. Electrons

travel in spaces located around a nucleus of protons and neutrons.

These spaces are about 10,000 times larger than the nucleus.

electron

neutron

proton

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